1. Field of the Invention
This invention relates to hydraulic devices and more particularly it concerns a novel hydraulically driven hammer system.
2. Description of the Prior Art
Fluid driven pile driving hammers are known in the prior art. Examples of these are shown in U.S. Pat. Nos. 3,298,447; 3,417,828 and 3,431,986. These pile driving hammers have a piston and cylinder arrangement either attached to or incorporated with a massive moveable ram. Hydraulic fluid is switched by valve means to flow into and out from the cylinder on opposite sides of the piston. In this manner reciprocal movement is produced between the piston and cylinder; and this drives the ram up and down. The switching of the hydraulic valve means in some of these prior art systems is obtained by exposing a pilot conduit to a surface over which a land separating regions of different pressure moves. In this prior art systems this land is the piston itself and the pilot conduit opens into the cylinder at a location over which the piston passes. As the piston moves over the pilot conduit opening the pressure to which the pilot conduit is exposed changes and this pressure change is sensed and used to switch the hydraulic valve means.
A difficulty arises in the prior art due to the fact that along the length of the piston or land which separates regions of different hydraulic pressure there exists a pressure gradient; and the precise location at which the pressure level causes switching is therefore not ascertainable. A further difficulty arises from the fact that the prior art systems require separate pilot conduits and associated pilot systems to produce switching at both ends of the hammer stroke. Moreover, since the hydraulic switching itself causes pressure changes in the region being sensed by the pilot conduit, rather elaborate arrangements were necessary in the prior art to prevent further switching in response to these pressure changes.